1. Field of the Invention
The present invention relates to a memory card device that can be used in various types of electronic apparatuses. More particularly, the invention relates to a memory card device that includes a clock generator.
2. Description of the Related Art
In recent years, various portable electronic apparatuses have been developed. Among them are personal computers, PDAs, digital cameras, mobile telephones. Memory cards, which are removable memory devices, are used in these portable electronic apparatuses. Two types of memory cards are known. The first is a PCMCIA card (generally known as “PC card”). The second is a SD (Secure Digital) card that is smaller than the PCMCIA card.
The SD card incorporates a flash memory. It is small and can yet store as much data as desired and operate at as high a speed as desired. The SD card has an improved 9-pin interface. Of the nine pins, four serve to transfer data to the host apparatus. Despite a few interface pins it has, the SD card can transfer data in sufficient performance.
Recently it is demanded that power consumption be reduced in small memory cards typified by SD cards. To reduce power consumption in an electronic device, the supply of clock signals to the internal core logic units of the electronic device may be stopped as is known in the art. In a device including a PLL (Phase Locked Loop), more power consumption can be reduced by stopping the PLL operation itself than by stopping the supply of clock signals from the PLL to the internal core logic units.
Once the PLL operation is stopped to set the device into power-saving mode, however, it will take much time to set the device return into the normal operating mode. This is because the PLL cannot generate stable clock signals for some time after it starts operating again. In other words, the internal core logic units cannot operate until the clock signals become sufficiently stable.
Particularly, a memory card that incorporates a nonvolatile memory such as a flash EEPROM cannot respond fast, because it takes a relatively long time to access the nonvolatile memory. To make the matter worse, the internal core logic units will need a long time to restart their operations once the PLL provided in the device is stopped to save power. The memory card inevitably responds even more slowly.